Skip to main content

Volume 54 Supplement 11

Special Issue: Slip and Flow Processes in and below the Seismogenic Region

Distinct S-wave reflectors (bright spots) detected beneath the Nagamachi-Rifu fault, NE Japan


Distinct phases reflected from mid-crustal reflectors (SxS and PxP phases) were observed in seismograms of aftershocks of 1998 M5.0 Sendai earthquake at nearby stations. We estimated the locations of the reflectors (bright spots) by using arrival time differences between these phases and direct waves. A clear reflector is located in the depth range of 15 to 21 km just beneath the fault plane of the M5.0 event. It dips toward the NNE direction with a dip angle of about 25°. Other reflectors are also located beneath the fault plane of the event. Internal structure of the S-wave reflector was estimated from spectral amplitude ratios of reflected SxS-wave to direct S-wave. Observed spectral ratios show that S-wave velocity in the reflector body is 1.1 km/s, and its thickness is about 50 m. This suggests that a thin reflector body (bright spot) partially filled with fluids exists in the lower crust beneath the focal area of the 1998 event.


  • Ansell, J. H. and E. G. C. Smith, Detailed structure of a mantle seismic zone using homogeneous station method, Nature, 253, 518–520, 1975.

    Article  Google Scholar 

  • Asano, Y., N. Umino, A. Nakamura, T. Okada, S. Hori, T. Kono, K. Nida, T. Sato, A. Hasegawa, M. Kosuga, and A. Hasemi, Spatial distribution of seismic scatterers beneath the Ou Backbone range, northeastern Japan, estimated by seismic array observations, J. Seism. Soc. Jpn., 52, 379–394, 1999.

    Google Scholar 

  • Hasegawa, A., A. Yamamoto, N. Umino, S. Miura, S. Horiuchi, D. Zhao, and H. Sato, Seismic activity and deformation process of the crust within the overriding plate in the northeastern Japan subduction zone, Tectonophys., 319, 225–239, 2000.

    Article  Google Scholar 

  • Hori, S., N. Umino, Y. Asano, and A. Hasegawa, S-wave reflectors in the crust of northeastern Japan, Programme and Abstracts The Seismological Society of Japan, 1999 Fall Meeting, p. 140, 1999.

  • Horiuchi, S., A. Hasegawa, A. Takagi, A. Ito, M. Suzuki, and H. Kameyama, Mapping of a melting zone near Mt. Nikko-Shirane in northern Kanto, Japan, as inferred from S × P and S × S reflections, Tohoku Geophys. J., 31, 43–55, 1988.

    Google Scholar 

  • Inamori, T., S. Horiuchi, and A. Hasegawa, Location of mid-crustal reflectors by a reflection method using aftershock waveform data in the focal area of the 1984 Western Nagano Prefecture earthquake, J. Phys. Earth, 40, 379–393, 1992.

    Article  Google Scholar 

  • Matsumoto, S. and A. Hasegawa, Distinct S wave reflector in the midcrust beneath Nikko-Shirane volcano in the northeastern Japan arc, J. Geophys. Res., 101, 3067–3083, 1996.

    Article  Google Scholar 

  • Nakajima, J., T. Matsuzawa, A. Hasegawa, and D. Zhao, Three-dimensional structure of Vp, Vs and Vp/Vs beneath northeastern Japan: Implication for arc magmatism and fluids, J. Geophys. Res., 106, 21843–21857, 2001.

    Article  Google Scholar 

  • Nakamura, A., Y. Asano, and A. Hasegawa, Estimation of deep fault geometry of the Nagamachi-Rifu fault from seismic array observations, Earth Planets Space, 54, this issue, 1027–1031, 2002.

    Article  Google Scholar 

  • Okada, T. and A. Hasegawa, Activity of deep low-frequency microearthquakes and their moment tensor in northeastern Japan, Bull. Volcanol. Soc. Jpn., 45, 47–63, 2000 (in Japanese with English abstract).

    Google Scholar 

  • Umino, N. and A. Hasegawa, Three-dimensional Qs structure in the northeastern Japan arc, Bull. Volcanol. Soc. Jpn., 37, 217–228, 1984 (in Japanese with English abstract).

    Google Scholar 

  • Umino, N., T. Okada, and A. Hasegawa, Foreshock and aftershock sequence of 1998 M5.0 Sendai, northeastern Japan, earthquake and its implications for earthquake nucleation, Bull. Seism. Soc. Am., 2002 (in press).

Download references

Author information

Authors and Affiliations


Corresponding author

Correspondence to Norihito Umino.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Umino, N., Ujikawa, H., Hori, S. et al. Distinct S-wave reflectors (bright spots) detected beneath the Nagamachi-Rifu fault, NE Japan. Earth Planet Sp 54, 1021–1026 (2002).

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


  • Fault Plane
  • Focal Mechanism
  • Bright Spot
  • Uppermost Mantle
  • Arrival Time Difference